Volume control for radio and interphone circuit



J. C. COE

Oct. 7, 1941.

VOLUME CONTROL FOR RADIO AND INTERPHONE CIRCUIT 'Filed NOV. 2, 1940 7Sheets-Sheet 1 HSI HSl

INVENTOR JAMES C. COE

ATTORNEY J. C. COE

VOLUME CONTROL FOR RADIO AND INTERPHONE CIRCUIT Filed Nov. 2, 1940 7Sheets-Sheet 2 HSI INVENTOR JAMES C. COE

ATTORN J. C. COE

Oct. 7, 1941.

VOLUME CONTROL FOR RADIO AND INTERPHONE CIRCUIT Filed Nov. 2, 1940 7Sheets-Sheet 3 INVENTOR JAMES c. 00E

ATTORNEY Oct. 7, 1941. J, CQE 2,257,731

VOLUME CONTROL FOR RADIO AND INTERPHONE C IRCUIT Filed Nov. 2, 1940 7Sheets-Sheet 4 E2 R4 Rd.

IIEM .E B

INVENTOR JAMES C. COE BY l c ATTO EY Oct. 7, 1941. J. c. COE 2,257,731 1VOLUME CONTROL FOR RADIO AND INTERPHONE CIIjUIT Filed Nov. 2, 1940 ISheets -Sheet 5 v IElL1' E D INVENTOR JAMES C. COE

ATTOR EY 0a. 7, 1941. J, c, CQE 2,257,731

VOLUME CONTROL FOR RADIO AND INTERPHONE CIRCUIT Filed Nov. 2, 1940 7Sheets-Sheet 6 R2 Rb v *vW HS El Ra Re HSa.

, I 1% ll R2 7 Rb WW Rl a . "'J --n R3 R El Re c H50 R4 E2 Rd,

IE :1. E: ii. A

HSa.

INVENTOR JAMES C. COE

BY l

ATTORiEY 0c 1, 1941. J. c. COE 2,251,731

VOLUME CONTROL FOR RADIO AND INTERPHONE (EIRCUIT Filed Nov. 2, 1940 7Sheqts-Sheei 7 R Ra.

HSI El INVENTOR JAMES c. COE

ATTO EY Patented Oct. 7, 1941 UNITED VOLUME CONTROL FOR RADIO ANDINTERPHONE CIRCUIT James C. Coe, Arlington, Va.

Application November 2, 1940, Serial No. 363,982

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) 22 Claims.

This invention relates specifically to communication systems employing anumber of sound reproducing devices and consists particularly of circuitconnections wherein a substantially constant impedance network isprovided in the voling apparatus, or interphone communication circuit.At the present time there are a number of attenuation control systemsfor radio receiving devices or the like, known to the art, which presenta constant impedance to the receiver output, but which are undesirablein aircraft installations because of their size and weight. These priorart attenuation control systems are illustrated by the U. S. patents ofDe Forest, 1,892,935, and Doran, 1,883,624. It is, therefore, an objectof the present invention to provide in a communicating system foraircraft having an individual and independent attenuation control systemfor each sound reproducing device connected thereto of light weight andreduced size so as to be particularly adapted for use in aircraft.

It is a further object of this invention to provide in a communicationsystem for aircraft having superimposed interphone and radio signals, avariable audio attenuator which will permit independent adjustment ofthe radio level in the various headsets and at the same time permit thereception of the interphone signal in all of the headsets.

It is also an object of my invention to provide a communication systemhaving a plurality of headsets connected to different radio receivingapparatus, each with independent attenuation control, and with a commoninterphone communication system connected to all the headsets.

It is also a further object of this invention to provide a communicationsystem having superimposed interphone and radio signals with individualvolume control of the radio signal to one or more headsets, which willproduce no change in the interphone level to the same headset.

It is an object of this invention to provide a system of communicationwhich has at least two independent signal sources, the outputs of whichare superimposed upon a number of sound reproducing devices, such asheadsets or loud speakers, with an individual audio attenuation controlfor the reproducing devices in the output circuit of one of said signalsources which permits that signal level in that reproducing device to bevaried from a maximum to a minimum without producing any noticeableeffect in the signal level from the same source upon the otherreproducing devices connected in parallel therewith through theirattenuating network, and without interfering with the reception of thesignal from the other sources in the same headset, although its levelmay be affected somewhat.

It is also an object of my invention to provide a communicationsystemhaving superimposed interphone and radio signals with anindividual volume control for the reproducing devices conneoted thereto,the volume control being arranged so that if the radio signal isincreased the interphone signal is automatically decreased, and viceversa.

It is also an object of my invention to provide a communication systemhaving both radio-and interphone signal sources, with volume controlmeans which permits the volume to be adjusted so that the level fromboth the radio and interphone source is increased or decreased together.

It is a further object of this invention to provide a communicationsystem having a plurality of independent signal outputs with pluralheadsets and individual attenuation controls therefor, arranged so thatthe signal from both sources may be received in one headset, while thesignal from only one source may be audible in the other headset.

With the foregoing and other objects in view, the invention consists inthe construction, combination and arrangement of parts hereinafterdescribed and illustrated in the drawings, in which:

Fig. 1 is a wiring diagram showing the connections for a plurality ofheadsets in parallel with the output of a radio receiver, amplifyingdevice or the like, shown symbolically as collector rings;

Fig. 2 is a wiring diagram showing a modification of Fig. 1;

Fig. 3 is a wiring diagram of a further modification of Fig. 1;

Fig. 4 is a wiring diagram of the connections shown in Fig. 1,illustrating how the circuit may be expanded to accommodate more thantwo headsets;

Fig. 5 is a wiring diagram showing the connections illustrated in Fig.1, with the addition of another sigiialsourcfindicated generally ascollector rings "and'connected to the same headset loads;

Fig. 6 shows a modification of Fig. 5, wherein independent signalsources are provided for each headset load in place of one of the commonsignal source illustrated by Fig. 5;

Fig. 7 is a. modification of the circuit arrangement shown in Fig. 6,wherein an additional headset load and its volume control circuitisadded to the output of one of the signal sources indicated in Fig. 6; V

Fig. 8 is a modification of Fig. .7, wherein a separate signal source isadded to supply the ad ditionalheadset load-of;Fig. 7;

Fig. 9 shows a-circuitarrangement similar to Fig. 5, with the :adidtionof a variable resistance in theoutput circuit ofthe added signal sourceof Fig. 5; I i

Fig. 9A showsa modification of the circuit arrangement illustratedinFig. 9, showing the contact arms of the variable resistance elementsconnected together mechanically and arranged so that an increase in theseries resistance from one signal source, causes a decrease in theseries resistance. in the output circuit of the other signal source; r

Fig. 9B is afurther modification of Fig. 9A;

Fig.9C is a. modification of the control circuit illustrated in Fig.9,arranged so that an increase or decrease in the series resistance fromone signal source will also cause an increase or decrease-in the seriesresistance of the other signal source;'

Fig. 9D is a modification of Fig. 90;

Fig. 10 is a modification of Fig. 1;

Fig. 11 is a modification of Fig. 1, showing different variableresistance elements with the contact arms of each connected togethermechanically;

Fig. 11A is a modification of Fig. 11, showing the addition of anothercommon signal source with a variable resistance in the output circuit.The contact arms of the series resistance ele ments in the differentsources are connected together mechanically, so that an increase ordecrease in one causes a decrease or increase in the other;

Fig. 113 is a modification of Fig. 11, showing circuit arrangementswherein the contact arms to the resistance elements connected in theoutput circuit of the different sources are arranged so that as theresistance connected in series with one source is increased ordecreased, the resistance connected in series with the other source isalso automatically increased or decreased;

Fig. 12 is a modification of Fig. 1;

Fig. 13 illustrates circuit connections showing an arrangement whereinthe output from two signal sources may be received in single headset,while the output ofonly one of the signal sources may be audible in theother headset; and I Fig. 14 is a modification of the control circuitsof Fig. 13. V

This invention in its broadest and most simple form is illustratedgenerally by Figs. 1, 2, 3 10, 11 and 12. In Fig. 1 there is shown asuitable source of E. M. F. illustrated diagrammatically as a pair ofcollectorring's; however, in actual practice this E. M. F. source may beany source of power, such as a transformer winding or the output circuitof a radio receiving apparatus, or interphone communicating system. Thetwo controlled load devices shown symbolically in the illustratedembodiments as headsets HSI and HSa, are each connected to this sourceof E. M. F. E I through a variable series resistor R2 and Rb,respectively. A second resistor R3 and Re is connected in shunt acrosseach headset load HSI and HSa. These resistors R3, Rc, R2 and Rb are allvariable resistance elements and may be of the type known to the tradeas a taper potentiometer. In determining the value of resistance R3 orR0 the designer must take into consideration the characteristics oftheir particular headset or other load. For example, if the impedanceratio R3 to HSI or R0 to HSa is too high, the control characteristicswill not be,

smooth, since the impedance of the network will then vary greatly withdifferent settings of the controls; while if the ratio is too low, anexcessive amount of incoming power is wasted in heating the shuntresistance elements R3 and R0. The proper impedance ratio is determinedbetween these two extremes to give a fairly constant impedance withsmall power loss.

The adjustable contact arm to the resistance element R2 is connected tothe point m, the junction point of the connections to RI, HSI and R3.Likewise the adjustable contact arm of the resistance element Rb isconnected to point n, which is also the junction point of the connectorsfor Rb, Re and H811. It is apparent from Fig. 1 that in one extremeposition of the contact arm of either resistance element R2 or R, therespective resistance is short circuited.

If resistors RI and Ra were not employed, the contact arms of thevariable resistance elements R3 and Re would be connected directly tothe output terminal of the source of E. M. F. El, so that in one extremeposition either of these contact arms would form a direct short acrossthe source El. In order to prevent such a direct short circuit, since itis an object of the invention to obtain a substantially constantimpedance network, the padding resistors RI and Ra are connected betweenthe source of supply EI and the contact arms of the various resistanceelements R3 and Re, respectively.

When the contact arms of R2 and R3 are in an extreme position where R2is entirely shorted out, and where the arm of R3 contacts point m, inthis position, RI will also be shorted out by the contact arm of R2.This constitutes the position of maximum power from El to the headsetload HSI since a circuit is completed from El to the contact arm of R2,to the parallel circuit consisting of the load HSI and the resistanceR3. The impedance of such a combination can readily be determined bymeans of computations known to the art. When the contact arms of R2 andR3 are in theopposite extreme position where the contact arm of R2contacts point m and the contact arm of R3 contacts the resistor R3 atthe end opposite the point m, in this extreme position, which is theposition of minimum power from El to the headset HSI, all of theresistance R2 is connected in series with the parallel circuitconsisting of the load HSI and its shunt resistance R3, while RI isconnected directly across this series parallel network. The impedance ofthe network consisting of HSI, RI, R2 and R3 can be determined byconsidering the group consisting of HSI and R3 in parallel, and thisgroup in series with R2, then with RI in parallel with the aforesaidseries parallel combination. The impedance of such a combination may bedetermined by means of computations known to the art. With anyintermediate setting the power available to HSI may be varied betweenextremes and the impedance of the group may be calculated by applyingKirchhofs laws.

The values of the RI, R2 and R3 may be chosen by means of computation asoutlined above, so that the desired conditions of maximum, minimum, orintermediate values of power to HSI may be obtained without producingexcessive change in the impedance presented to El due to changing thecontacts. Thus it is possible with the value of these resistanceelements properly computed to obtain a variation of power to HSI bychanging the controls to R2 and R3 without producing any appreciableeffect upon the power available and delivered to HSa.

In like respect the resistance elements Rb and Re may be similarlyadjusted without producing any appreciable eifect upon the poweravailable and delivered to HSI.

It should be understood that Ra corresponds in function to RI, Rb, toR2, and R to R3, and what has been set out above concerning RI, R2 andR3 applies also to Ra, Rb and Re.

The impedance network of Fig. 1, as well as the impedance network of theother embodiments of the invention herein presented, do not necessarilypresent an absolutely constant impedance network to El. In fact, it hasbeen found from actual practice that the change in the volume control'inheadset HSI, for example, from a maximum to minimum may produce avariation in the impedance of the combination HSI, RI, R2 and R3 of 3 to1 without producing any effect upon headset HSa, which would beappreciable to ordinary hearing.

The contact arms to R2 and R3 are in the preferred embodiment of theinvention mechanically coupled together so as to be moved in unison, or,in the parlance of the art, they are referred to as being ganged;similarly, the contact arms to Rb and Rc may also be ganged.

It is thus apparent that in the network shown in Fig. 1, the values ofthe resistance elements, may be properly selected so that the input toone headset may be adjusted within wide limits without changing theinput into the other headset by an amount which would be appreciablynoticeable to the average hearing.

The circuit arrangements shown in Fig. 1 may take the form shown also inFigs. 2, 3, 0, 1 and 12, wherein minor variations have been incorporatedin each circuit, which consists specifically in details in theconnections to the resistance elements comprising the control circuit ofeach headset load. For example, Fig. 2 is nearly identical with Fig. 1,except that the resistance elements R2 and Rb are not directly connectedto the points m and n.

In Fig. 3, the control arms for resistance elements R2 and R3 (Rb andR0) do not move in the same direction as they may in the aforementionedembodiment, since in this figure the contact arms for resistances R2 andRb are connected to their respective resistances at the end opposite thepoints m and n. Thus, in order to short out the resistances R2 and Rbthe respective contact arms of each are moved downwardly to the limit oftheir movement to point m or n, while in order to short out resistanceelement RI or Ra the contact arms for resistance elements R3 and R0,respectively, must be moved upwardly to the limit of their movement topoints m and n at the same time the other contact arms are at thesepoints. Thus, a different type of coupler must be used to gang" theseresistances than would be provided for the resistance elements in theabove embodiments.

In the modification illustrated by Fig. 10, the headset HSI and thecontact arm to resistance R2 are connected directly together withoutcontacting the point m. Similarly, HSa and the contact arm to Rb areconnected directly together without contacting the point n. This circuitpresents a'somewhat diiferent impedance network from that shown inFig. 1. For example, when the contact arms are adjusted to the positionof maximum power delivery to the headset HSI the resistance elements RI,R2 and R3 form a series parallel circuit in shunt across the headsetload HSI, with RI and R2 in parallel, and in series with R3. Thecomputations of the impedance network illustrated by the modificationmay be computed by means set out above.

Fig. 12 differs from Fig. 10 only in that R3 and R2 are not connectedtogether at the point m, and similarly Re and Rb are electricallyseparatedat point n. The impedance offered by this network for a settingwith the maximum power delivered to HSI may be computed by consideringthe parallel circuit formed by HSI in one branch, and RI and R3 inseries in the other branch, with R2 shunted out. In the position forminimum power delivery to HSI, R2 is in series with HSI and RI is inshunt with this series circuit.

Fig. 11 presents an arrangement of the various resistance elementswherein the contact arms of RI and R3, as well as the contact arms of Raand Re may be tied together electrically as well as mechanically. In theposition for maximum power delivery RI and R2 or Ra and Rb are shortcircuited by the contact arms of resistance elements RI and R3 or Ru.and Re. In any intermediate position a portion of each resistanceelement RI and R3 is connected in series and forms with R2 a parallelcircuit, the combination being connected in series with HSI. A closedshunt circuit may also be traced from the source EI through the portionof resistance RI and through a portion of the resistance R3 notconnected in the aforementioned mesh circuit. It is to be noted alsothat in this arrangement RI and R3 are variable while series resistor R2is fixed, distinguishing thereby from Fig. 1.

Fig. 4 presents a modification of the embodiment illustrated in Fig. 1,and differs therefrom in that an additional headset load HSA and itscontrol is provided. This circuit is merely illustrative of the mannerin which the circuit diagrams of Figs. 1, 2, 3, 10, 11 and 12 may beexpanded to accommodate more than two headset loads. The control for theadditional headset load comprises the resistance elements RA, RB and RC,each of which is connected in exactly the same manner as are resistanceelements Ra, Rb and R0 of Fig. 1, and each serve the same function. Theimpedance of each of these elements is also selected so that, as theinput to HSA is varied only a minor eifect will be produced upon thepower delivered to the other loads, which effect is not perceptible toordinary hearing.

Fig. 5 presents a somewhat different embodiment of the invention, thecircuit connections shown herein being similar to those shown in Fig. 1,except that a second source of E. M. F.

E2 is provided, one side of which is connected directly to El, the otherside being connected to each'of the headset loads HSI and HSa throughthe series resistance elements R4 and Rd, respectively. The two sourcesof E. M. F. are again illustrated diagrammatically as collector rings,but in one practical application to which this circuit arrangement isparticularly adapted, El is the output circuit of a radio receiver, andE2 the output of an interphone communicating system. Since in thismodification an additional source of power is provided, it is necessaryin order to prevent the adjustment of the attenuation to one load fromafiecting the power level in the other load to provide isolatingresistors in the circuit connecting this source to the load devices. InFig, 5 (for illustrative purposes only) the contact arms of R2 and R3are adjusted for maximum power delivery to HSI while the contact arms ofRb and Re are adjusted for minimum power delivery to HSa. In thisposition of adjustment the voltage drop from El to m is obviouslydifierent than the voltage drop from El to 11., since in the attenuationnetwork for HSI, RI and R2 are shorted out and the potential oi. m issubstantially that of El; therefore points m and n are at substantiallydifferent potentials.

The conductors connecting E2 to each of the load devices would obviouslyform a low impedance path for circulating current between points m and12. With the attenuation to 118:: high the path for this circulatingcurrent would also include H811. and any further adjustment of theattenuation to HSI would change the potential between points m and n,thereby changing the value of the circulating current and afiecting thepower level in HSa. In order to reduce the value of the circulatingcurrent in the closed circuit formed between the points m and n by theconductors connecting E2 to the respective headset loads, the isolatingresistors R4 and Rd are inserted in series in this closed path, as

is apparent from Fig. 5. Were it not for these isolating resistors powerfrom El could pass by means of the shunt path around the resistor R2through the conductors connecting E2 to headsets HSI and HSa onto theheadset HSa, thereby forming a shunt path around the attenuating networkformed by the resistance elements Ra,

Rb and Re, and the power level in HSa would be aifected by the change inthe setting of the attenuation in the circuit to HSI. The circuit formedby R4 and Rd with E2 comprises a rejection circuit which prevents thepower of El from reaching Hsu and R4, Rd introduce attenuation betweenpoints n and HSI.

In an actual application wherein El is a radio receiver output and E2 isthe output of an interphone communicating circuit, the value of theresistance elements are selected so that upon decreasing the level ofthe radio signal to KS; to a minimum, by means of the adjustable contactarms of resistance elements Rb and Re, a greater response from theoutput of the interphone communicating circuit will be produced in theheadset HSa. Conversely, increasing the radio receiver response in HSIby the contact arms to R2 and R3 results in a reduced response in theheadset HSl from the output of the interphone communicating circuit. Itshould also be apparent that this circuit incorporates the same featuresof Fig. 1, namely, the adjustment of the output level of either headsetwill produce no noticeable effect upon the output level E2 across theterminals of the headset HSI.

to the other headset from El. In general, it

may be stated that an adjustment or the controls which results in adecrease in the radio response to a particular headset, results also inan increase in the interphone response in that headset but produces noperceptible change in the other headsets connected to the radio, andconversely an increase in the radio response in a headset results in adecreased interphone response in that headset, but produces nodetectable change to ordinary hearing in other headsets connected to theradio output.

Assuming that the contact arms to Re and Rb are fixed, and that thesliding contacts to R2 and R3 are placed in position so as to result inminimum power delivered to HSI from El, in which position Rl isconnected to R3 at the end opposite point 112 and all of R2 is insertedbetween point m and El, the impedance oflered E2 by the left-handportion (Fig. 5) of the circuit consists of the following seriesparallel arrangement of resistance elements, namely, R4 in series with aparallel circuit comprising the following paths, R2 in series with theparallel circuit comprising El and RI, R3 and the headset I-ISI. It isto be noted that R2 then serves in a measure to isolate E2 from El andpresents a high impedance circuit to the power source In this controlposition the maximum power is delivered from E2 to HSI.

When the sliding arms to R2 andR3 are placed so as to cause El todeliver maximum power to HSI, then the power from E2 to HSI is less,because in this position RI and R2 are short circuited and the addedload presented by El, Ra, Re, Rb and H511 forms a low impedance shuntpath across the headset I-ISl when the controls to HSa are set forminimum power. The impedance presented to E2 by the lefthand portion ofthe circuit may be calculated by considering the following seriesparallel circuit, R4 in series with a parallel circuit comprising thefollowing paths, El, R3 and HSI, or Ra when the control to HSa is notset for minimum attenuation to El.

Thus it is evident that adjusting the contact arm to R2 and R3 to givemaximum signal level in HSI from El is accompanied by a minimum signallevel in HSI from E2; and, conversely, adjusting the contact arms to R2and R3 to give a maximum signal level in HSI from El is accompanied by amaximum signal level in HSl from E2.

The amount of change of interphone response accompanying a change inradio level adjustment, as above described, is dependent upon therelative values of R4 and Rd to the impedance of El since the bestregulating characteristics are available when the ratio of external tointernal impedance is high; therefore, if R4 and Rd are of a high valuethe range of change is greater than for lower values of R4 and Rd. Ifthese values are too .low the rejection characteristics of the R4, Rdand E2 combination are lessened, and El is loaded more heavily thereby.In general, rejection characteristics are improved by lowering theimpedance of E2 and increasing R4 and Rd.

Changing the receiver output El by other means such as in the receiveritself without changing the individual volume control has practically noeffect upon the interphone response in either headset; however, if theradio receiver output is low such as occasioned by a weak incomingsignal which necessitates setting the individual volume control so thatthe attenuation between the receiver output and the controlled headsetis a minimum, the interphone level to that headsetis not as high as whena much greater receiver output is available, which would permit anintermediate setting of the controls. In the latter eventattenuation isintroduced between the radio receiver output and the controlled headsetin order to reduce the radio signal to a more comfortable level and thisis automatically accompanied by an increase in interphone output to thatheadset.

It should be apparent that the additional source of E. M. F. E2 and theresistance elements R4 and Rd may also be added to the modifications, asillustrated by Figs. 2, 3, 10, 11 or 12, just as well as to Fig. 1.

Fig. 6 presents a variation of the embodiment illustrated in Fig. 5,where separate E. M. F. sources El and E3 are provided for each headsetHSI and HSa in place of the single source El shown in Fig. 5. Themodification herein illustrated has a specific application in aircraft,for example, where the persons wearing headsets may listen to twoseparate incoming radio signals (El and E3) individually adjusted to anylevel between maximum and minimum and yet both can listen to a commoninterphone signal, the level of which is relatively independent of thesettings of the volume controls which adjust the individual radio signallevels for the separate headsets. However, the circuit may be designedso that as the level of the radio signal to one headset is decreased byits individual control, the level of the interphone to that headset isincreased slightly, and conversely, as the radio output is decreased toone headset, the interphone level in that headset is increased, as wasexplained in connection with Fig. 5. This feature may be of advantagewhen a weak radio signal is being received because only then would it benecessary to adjust the individual volume level for minimum attenuationof the radio signal, and under those conditions a relatively weakerinterphone level is desirable; and conversely, if the volume level isset near maximum attenuation of the radio signal to that headset, theradio receiver output is either very great or not desired at all in thatheadset, and a greater interphone level is useful.

The proper functioning of the device depends to a certain extent uponproper setting of the receiver output control or receiver sensitivitycontrol, or both, by means not included in the portion of the circuitshown.

The modification illustrated by Fig. 7 includes in addition to thecircuit arrangement shown in Fig. 6, an additional headset load HSA andits control circuit which includes the resistance elements RA, RB andRC. This figure is illustrative of how Fig. 6 may be expanded toaccommodate additional loads somewhat in the same manner in which Fig. 1was expanded into Fig. 4. In addition to the control resistors RA, RBand BC, the isolating resistor RD is also provided. This resistor RDserves a function similar to resistors R4 and Rd, in that it serves toreduce the voltage from E2 to HSA and presents, with R4, a highimpedance path between the points and m, and thus serves as a rejectioncircuit. Fig. 7 also differs from Fig. 6 in that a capacitor Cl isconnected across the power supply E2.

to improve its rejection characteristics. As ex- This capacitor isinserted in this circuit plained above, E2 may be the secondary windingof a transformer having a relatively high impedance to high frequencycurrents. This is undesirable since, as was pointed out above, theimpedance ratio of R4 to E2 should be high in order to give the bestrejection. The effect of Cl is to offset the high impedance offered byE2 since the impedance of Cl decreases as the frequency increases. Insome applications Cl is replaced by a band pass filter.

The circuit shown by Fig. '7 may be modified to include a separatesource of power supply for the headset load HSA; such a modification isillustrated by Fig. 8.

{is noted above, this invention was designed primarily for use inaircraft installations where both radio and interphone communicationsare provided, although it is not particularly restricted to such anarrow field. In such an application, however, a number of headsets areusually provided having a common interphone output, with a variablenumber of radio receivers, and in such an installation it is importantthat the number of headsets for each radio receiver be changed asconditions require, without changing the interphone level to any of theother headsets. Furthermore, it is also an important feature of thisinvention to have adjustments of the attenuation in the output of theradio receiver produce certain desirable changes in the interphonesignal level in the common headset load, the said changes beingdependent upon the particular installation, that is, in someinstallations one load station, HSI, for example, may be primarilyinterested in receiving the radio signal; the interphone signal beingonly of secondary importance. The operator at the other load station,HSa for example, may be more interested in receiving the interphonerather than the radio signal; consequently the variable attenuator mustbe constructed so that the attenuation to one source will be increasedwhile the attenuation to the other source will be decreased. Forexample, the audio attenuation to HSI for radio is reduced so that aweak incoming radio signal may be heard, however, the signal level fromthe interphone output must be reduced at the same time so that it wontinterfere with the reception of the weak radio signal. Under certainother conditions it may be desirable to have the interphone level remainfixed while the attenuation to the radio signal is being adjusted orchanged. In certain other installations it is desirable to have thelevel of the radio signal and interphone signal increase or decreasetogether by means of a single control. Thus, the signal level in bothradio and interphone may be increased or decreased as the noise level atthe particular load station changes. The above features may be attainedby means of circuit arrangements of Figs. 9 to 9D, inclusive.

Fig. 9 is similar to Fig. 5, except that additional resistance elementsR5 and Re are provided in the rejection circuit in series with R4 or Rd,respectively. As in the other modifications, the primary object of R2and R3 is to permit the adjustment of current to HSI from El withoutproducing any noticeable effect upon the level in HSa or other headsets;the primary function of R5 is to provide an adjustment of the impedancein the circuit from E2 to HSI and also to limit the circulating currentfrom El which may reach HSa through the resistance elements R4, Rd andRe.

The contact arms of the resistance elements R2, R3 and R5, as well asthe resistance elements Rb, Re and Re, may be ganged so that when thecontact arms for R2 and R3 are set in a position for maximum powerdelivery to HSI, in which position the resistance elements R2 and RI areshort circuited, the contact arm to the resistance element R5 will alsobe moved to a position wherein all or substantially all of theresistance remains in series with the headset load HSI and with R4. Thisis the position of minimum power delivery from E2 to HSI. It isapparent, therefore, that the resistance elements R5 and Re tend toaccentuate the inherent characteristics of the circuit arrangements ofFig. 5, namely, an increase in the output level to a headset from onesource, produces a decrease in the output level to that same headsetfrom the other source, and vice versa.

The arrangement shown in Fig. 9A is the same as far as circuitconnections are concerned, as the arrangement in Fig. 9, except that thecontact arms R2 and R5 are connected together electrically as well asmechanically. The resistance elements R5, R4, Rd and Re, as well as thesource of E. M. F. E2, serve as a rejection circuit in the same manneras previously discussed in connection with Fig. 5. With the control toHSI set for minimum attenuation, and with the control to HSa set formaximum attenuation, the potential differential between points m and nis a maximum. In this position, however, the resistance element R5 is inseries with R4 and. Rd. The series resistors R5 and R4 reduce thevoltage from the source El that may appear across E2, and since thissource E2 may have a low impedance as compared to the impedance of Rd(and Re) the voltage is still further reduced to HSa. Consequently, withthis circuit arrangement the output level from El to HSI may be adjustedto a high value, while the output level from El to HSa may at the sametime be adjusted to a low value. When the control to HSI is set formaximum power level from El, as illustrated, R4 and R5 are in serieswith HSI and the voltage from E2 across HSI is reduced to a minimum;furthermore, R2 is short circuited so that El and the control circuitfor HSa presents with HSa a substantially constant impedance shunt pathacross the headset load HSl for the power delivered from E2. Since R5 isin series with E2 and HSI the rejection characteristics of E2 areimproved because the ratio of R4 and R5 to E2 is increased and the valueof the current reaching the shunt path formed by E2 is reduced.

In the position for minimum power delivery from El to Sc, Rb isconnected to El in series with a. parallel circuit comprising HSa andRe. The resistance element Ra is connected directly across El inparallel with the above mentioned series parallel connection. Theimpedance offered to El by this circuit is substantially constant butthe attenuation to HSa from El is maximum. In this control position thecontact arm for Re is moved to a position wherein the resistance isentirely short circuited so that the attenuation ofiered to E2 is aminimum, and the resistance of the control circuit for HSa is high, sothat the maximum power delivered by E2 to this portion of the circuit isabsorbed in the load device HSa.

It is therefore apparent that the natural inverse eflect considered inconnection with Fig. 5 is accentuated by the addition of R5 and Re. Thatis, if the level in HSI from El, for example, is increased, the level inHSI from E2 is decreased further by the addition of R5 together with itscontrol.

The circuit arrangement shown in Fig. 9B appears to be similar to thearrangement shown in Fig. 9A. However, it should be noted that thejunction point of R2 and R5, as well as Rb and Re, is not connected tothe contact segment of the contact arms for R5, R2 or Rb and Re,respectively. For any intermediate setting, therefore, the radio signalEl may take a path through all of R2, or through only a portion of R2 inseries with a portion of the resistance element R5, or also through RIand a portion of R3 to m, and similarly the interphone signal E2 maytake a path through all of R5 or through only a portion of R5 and aportion of R2, or through the other part of R2, the part of R3 leadingto the point m to the headset, or in shunt across the headset throughthe other portion of R3.

As was pointed out in connection with Fig. 9, the control resistanceelements R2, R3 and R5, as well as control resistance elements Rb, R0and Re, may be adjusted individually. In one extreme position ofadjustment Rl R2 and R5 may all be short circuited. This wouldconstitute a position of maximum power in HSI from E2; while in theother extreme position of the controls HSI receives minimum power fromboth El and E2. Fig. presents a circuit arrangement wherein the contactarms of these resistance elements may be ganged to produce a similarefiect. That is, resistance elements R5 and Re in this modificationcompensate for the inherent inverse effect naturally present in thecircuit, rather than to accentuate this effect, as was the case in theembodiment presented by Figs. 9 to 93, inclusive. The values of theimpedance elements may also be selected so that the level from thesource E2 will remain substantially constant (or increase and decreaseslightly) with an increase or decrease in the level from the source El.

As illustrated in Fig. 90, the control arms for the variable attenuatorto HSI are positioned so that R2 and RI are shorted out by the controlarm of R2, and HSI and R3 form a parallel circuit connected across thesource El. This is the position of minimum attenuation and maximum powerdelivered from El to HSI. The control arm to the resistance element R5is also moved to a position wherein the resistance of R5 is alsoshorted, thereby reducing the attenuation introduced in the circuit fromE2 to HSI. The reduction of the attenuation introduced in this circuitcompensates for the increase in shunt load presented by El and Ra, whichform a low impedance path across the headset HSI if the attenuatingcontrol to HSa is set for minimum power in HSa from El. HSa and itscontrol circuit also present an additional load. These loads are ineffect isolated from E2 through R4 when the attenuation to HSI from Elis maximum, since for this position R2 is connected in series therewith.The efiect of the resistance R5 is to compensate for the inherentinverse effect referred to above; that is, as the attenuation from El toHSl is increased the attenuation from E2 to HSl is also increased;therefore, the signal in HSI from E2 may remain at a constant level orincrease and decrease slightly with an increase and decrease in thesignal level in HSI from El by means of the audio attenuator.

Fig. 9D difiers from Fig. 90 in that R5 and R3 are notdirectlyelectrically connected to HSI and HSa except through theircontact arms. In this circuit power from E2 in order to reach HSI may gothrough R2 and RI as well as R5, except in the extreme position of thecontact arms.

This modification is similar to the arrangement shown in Fig. 93, exceptthat in this arrangement by proper choice of resistor values, the levelto HSI from El may be adjusted between extremes without producing anyeffect in the level in HSI from E2, and similarly for HSa.

Fig. 11A differs from Fig. 11, as described above, by the addition ofanother source of power, E2, fixed resistors R4 and Rd, and variableresistors R and Re. The contact arm to R5 has a mechanical but not anelectrical connection with the contact arms to RI and R3. The contactarm to Re has af similar mechanical connection to the contact arms Raand Re. These mechanical connections are shown in dotted lines. Thecontact arms to RI and R3 are tied together electrically as well asmechanically; similarly, the contact arms to Ra and Re are tied togetherelectrically as well as mechanically. When the contact arms to RI, R3and R5 are adjusted so that maximum power is delivered from El to HSI,then minimum. power is delivered from E2 to HSI; and, conversely, ifthese contact arms are adjusted so that minimum power is delivered fromEl to HSI, then maximum power is delivered from E2 to HSI, otherconditions remaining fixed. A movement of the contact arms of RI, R3 andR5 in one direction increases the power in HSI from El, and decreasesthe power in HSI from E2, or vice versa. By changing the position of thecontact arms to RI, R3 and R5 the power from El to HSI and from E2 toHSI is changed, yet the impedance of the load on El due to RI, R2, R3and HSI may be relatively constant if the values are properly chosen.Resistors Ra, Rb, Rc, Rd, and Re perform the same functions with respectto HSa as the corresponding resistors RI, R2, R3, R4 and R5 perform withrespect to HSI.

Fig. 11B differs from Fig. 11A in that R5 and Re are connected in adifferent manner, such that moving the contact arms to RI, R3 and R5,forexample, so as to increase the power delivered to HSI from El, alsoresults in an increase in power delivered to HSI from E2; and,conversely, a movement of the contact arms to RI, R3 and R5, whichdecreases the power delivered to HSI from El, results also in a decreasein the power delivered to HSI from E2, provided the various values ofthe resistance elements have been properly chosen, and other conditionsremain fixed. In an extreme case, if R5 had practically no resistance,moving the contact arms of RI, R3 and R5 to increase the power from Elinto HSI would result in a decrease in power available from E2 to HSI,because the effective resistance between EI and E2 would be lowered andEI would absorb more power from E2 at the expense of HSI than when thereis a greater value of resistance between El and E2. If the contact armsto RI and R3 were adjusted so as to practically short circuit R2, thenHSI and El are practically in parallel and El absorbs a maximum of powerfrom E2. Conversely, if the contact arms to RI, R3 and R5 are adjustedfor minimum power delivered to HSI from. El, wherein R2 is in serieswith HSI, and RI and R3 each form a shunt path connected in paralleltherewith. In this control position E2 would not only have theresistance of R4 between it and El but would also have all theresistance of R2, so the power of E2 absorbed in El would be less, or E2would be loaded comparatively lightly. If the value of R5 is properlychosen, the amount of power from E2 into HSI will remain relativelyconstant regardless of the power into HSI from El, 1. e., regardless ofthe position of the controls to RI, R3 and R5. In other words, R5 actsas a compensating resistor whose contact arm is moved in unison with thecontact arms to RI and R3, the active portion of R5 having just theright value of resistance to compensate for th variable load presentedto E2 due to changing power from El to HSI. When the load on E2 is theheaviest, the combined resistance of the active portion of R5 and R4 isminimum, thereby maintaining a relatively constant voltage from E2across HSI, regardless of the voltage from El across HSI, or the powerinto HSI from El.

In Fig. 13 there is shown a circuit in which headset HSa is connected tothe output of receiver #2 through the control consisting of threeresistors Ra, Rb and R0. The headset HSa also receives power from theoutput of receiver #1 through the control consisting of resistors RI, R2and R3. The headset HSI also receives power from the output of receiver#1 through the control consisting of resistors Rx, Ry and Re. Inaircraft it is often desired to listen to or to monitor two radioreceivers simultaneously in one headset, while in a second headset itmay be desirable to listen to only one of these two receivers withoutundue interference from the second receiver. In Fig. 13 the signal fromreceiver #1 is desired in both headsets HSI and H811, while the signalfrom receiver #2 is desired only in headset HSa. In Fig. 13 the signallevel from receiver #2, is less in HSI than in HSa unless the controlsare adjusted for maximum signal, i. e., for minimum attenuation,Frequently the signal available from one receiver is such that it ispossible to obtain sufficient volume level where desired, with onecontrol adjusted for more than minimum attenuation. If one control isset for more than minimum attenuation, then the signal strength fromreceiver #2 is less in HSI, where it is not desired, than in 11511 whereit is desired. If, however, two or more controls are set for more thanminimum attenuation, each one introduces attenuation of the unwantedsignal from receiver #2 to the headset HSI. For example, Rb and Re incombination may be set to attenuate the signal from receiver #2 toI-ISa, and R2 and R3 may be set to attenuate the signal from receiver #1to HSa, also Ry and R2 may be set to attenuate the signal from receiver#1 to HSI. In such an arrangement the unwanted signal in HSI, namely,from receiver #2, is attenuated by three sets of controls, whereas thewanted signal is attenuated by only one set, and thus the attenuation ofthe unwanted signal to HSI has been attenuated due to each control. Theunwanted signal may be attenuated very little to HSI if the threecontrols are set for very little attenuation due to Weak signals fromboth receivers, or it may be attenuated very much to HSI if the threecontrols are set for considerable attenuation due to strong signals fromboth receivers. If the output of receiver #1 is high, necessitating thesetting of controls between it and the headsets so as to introduce someattenuation, then the unwanted signal from receiver #2 to HSI isattenuated by both of these controls, and the desired signal isattenuated by only one control. While this variable attenuation of theunwanted signal is not always desirable it is less objectionable than ifthe receiver outputs were both placed directly in parallel, in whichcase the attenuation of. the unwanted signal to a headset is always thesame as the attenuation of the desired signal. Furthermore, tying thetwo receiver outputs in parallel causes each one to load down the otherat the expense of the audio power available to the headsets. By means ofthis device the receiver outputs may be separated by means of thecontrols, one or more of which may be set for more than minimumattenuation, thereby preventing the re ceiver outputs from beingdirectly in parallel. In Fig. 13 the receiver outputsare directly inparall l only when the two volume controls connected between thereceiver outputs are set for minimum attenuation.

As previously indicated, audio attenuating networks which present anabsolute constant impedance regardless of their control position areknown to the art, also more accurate means for eliminating or reducingthe unwanted signal are also known, but these means, as known, cannot beadapted to aircraft communication systems because of their weight andthe space required. It is apparent that my invention involves acompromise between attenuators of a constant impedance, requiringadditional elements and complicated circuit connections, with thenecessity of added weight and additional space, and substantiallyconstant attenuating means involving less weight and space. The meansthat I have devised has proven, however, to be entirely satisfactory,since in aircraftcommunication the noise level'is high and slightchanges in level are not noticeable.

The modification shown in Fig. 14 is similar in functioning to Fig. 13,although differing therefrom in control details. The controls hereinemployed are similar to the circuit described above and illustrated byFig. 11.

Other modifications and changes in the numbet and arrangement of theparts may be made by those skilled in the art, without departing fromthe nature of the invention, within the scope of what is" hereinafterclaimed.

The invention described herein may be manufactured and/or used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

I claim:

1. In the output circuit or an aircraft radio receiving apparatus or thelike, the combination which includes, a pair of output terminals, aplurality of sound reproducing .devices connected thereto, means forvarying the volume of at least one sound reproducing device from amaximum to a minimum without causing a variation in the impedance acrosssaid output terminals of an amount greater than the ratio of one tothree, said means comprising fixed and variable impedance elements, oneof said variable impedance elements being connected in series with thesound reproducing device, a second variable impedance element forming ashunt path across the said output terminals and a fixed impedance beingconnected in series with said variable shunt impedance so that in oneextreme position the fixed resistor element will alone form a shunt pathacross said terminals.

2. In a system of the character described including a sound reproducingdevice connected across the output circuit of radio receiving apparatus,means for varying the volume of said reproducing device from a maximumto a minimum without varying the impedance across the output circuit anamount greater than the ratio of one to three, said means comprising aplurality of impedance elements, one of said impedance elements beingvariable and connected in shunt with said reproducing device, the otherimpedance elements comprising a mesh circuit including a fixed andvariable impedance element, said mesh circuit being connected in serieswith the said reproducing device, and means for shorting out theimpedance offered by said mesh circuit for one extreme position ofvolume control, said last named means comprising the adjustable contactarms for the variable impedance elements.

3. In combination, a plurality of sound reproducing devices each havingan output circuit, a variable attenuator in each of said circuits, saidattenuator having a single movable element, each of said attenuatorscomprising an impedance network, including a variable impedance inseries with the reproducing device, and a variable impedance in shuntacross said output circuit, said single movable element comprisingadjustable contact arms for each variable impedance, and meanscomprising a fixed impedance connected in said network whereby theimpedance of said network and reproducing device will remain substantially constant for all control positions.

4. In a communication system for aircraft having an output circuit of aradio receiving apparatus and an output circuit of an interphonecommunication system, a plurality of isolating resistance elements, aplurality of sound reproducing devices each connected in parallel to theoutput of said interphone communication system, each reproducing devicehaving in series therewith one of said isolating resistors, a variableattenuator for at least one sound reproducing device connected in theoutput circuit of said radio receiving apparatus to said reproducingdevice, said attenuator comprising an impedance network having fixed andvariable impedance elements, and means comprising the contact arms ofeach variable impedance element for changing the level in said soundreproducing device from a maximum to a minimum without producing achange in the combined impedance at the output terminals an amountgreater than the ratio of 1 to 3.

5. In a communication system comprising a plurality of audio frequencysupply means, a plurality of sound reproducing devices connected inparallel with each of said audio frequency supply means so that eachreproducing device has superimposed thereon signals from at least twosupply means, an audio attenuator for each reproducing device connectedin the output circuit of one of said supply means, an isolating resistorconnected in the output circuit of said other supply means, saidattenuator comprising an impedance network having variable impedanceelements connected therein, and means comprising the adjustable contactarms of said variable impedance elements whereby the impedance of saidelements may be adjusted so that the current from said one supply meanscan be changed from a maximum to a minimum and at the same time produceonly a slight inverse effect in the level of said reproducing devicefrom said other supply means.

6. In combination, a plurality of sound reproducing devices, an audiofrequency supply means for each sound reproducing device, an audioattenuator connected in the output circuit of the audio frequency supplymeans for at least one of said sound reproducing devices, a common audiofrequency supply means, conductors connecting each of said reproducingdevices to said common supply means and in parallel therewith, anisolating resistor in each of said conductors, said attenuatorcomprising an impedance network having fixed and variable impedanceelements, and means comprising the contact arms of said vari ableimpedance elements whereby the output level from one supply means tosaid reproducing device can be changed from a maximum to a minimum andat the same time produce only a slightly noticeable inverse effect inthe level of said reproducing device from said other supply means.

'7. In a communication system for aircraft having an output circuit of aradio receiving apparatus and an output circuit of an interphonecommunication system, the combination including plural sound reproducingdevices connected in parallel with the output circuit of the radioreceiving apparatus, a variable audio attenuator connected in the outputcircuit to each reproducing device whereby the reproducing devices mayoperate at substantially difierent levels, conductors connecting eachsound reproducing device in parallel with the output of said interphonecommunication system, and means comprising isolating resistors in theinterphone communication system output circuit whereby a highattenuation is offered to a radio signal reaching either reproducingdevice through a circuit including these conductors although the audioattenuators are set at different levels.

8. In a communication system comprising a plurality of audio frequencysupply means, a plurality of sound reproducing devices connected to eachof said audio frequency supply means, a variable audio attenuatorconnected in the output circuit to each reproducing device whereby thereproducing devices may operate at substantially different levels, acommon audio frequency supply means, conductors connecting eachreproducing device in parallel thereto, a plurality of isolatingresistors connected in the output circuit of said common audio frequencysupply means and in series with each reproducing device, and meansforming a low impedance shunt path to high frequency currents acrosssaid common supply means, said last named means and the isolatingresistors forming a. voltage divider thereby preventing a signal fromone independent supply means from reaching a reproducing deviceconnected to another supply means by way of said conductors.

9. In a communication system having a plurality of radio receivers andan interphone communication system, the combination including an outputcircuit for each radio receiving apparatus, and an output circuit forthe interphone communication system, a plurality of sound reproducingdevices each connected to an output circuit of a radio'receivingapparatus, a variable audio attenuator connected in the output circuitto each reproducing device whereby the reproducing devices may operateat substantially different levels, conductors connecting each soundreproducing device in parallel with the output of saidinterphonecommunication system, and means comprising a plurality of isolatingresistors connected in the output circuit of said interphonecommunication system and in series with each sound reproducing devicewhereby a high attenuation is offered to a radio signal from the outputof one receiver reaching a reproducing device connected to a differentradio receiver output through a circuit including these conductors.

10. In a communication system having a plurality of sound reproducingdevices, an independent audio frequency supply means for each soundreproducing device and a common audio frequency supply means for all ofsaid sound reproducing devices, a variable audio attenuator in theoutput circuit of the independent audio frequency supply means for eachsound reproducing device, said audio attenuator comprising an impedancenetwork having fixed and variable impedance elements which together withthe reproducing device offer a substantially constant impedance to eachindependent audio frequency supply, so that the signal level in eachsound reproducing device from the common audio frequency supply sourcewill not be lost as a result of the adjustment of said variable audioattenuator.

11. In a communication system for aircraft having an output circuit of aradio receiving apparatus and an output circuit or an interphonecommunication system, a sound reproducing device connected across bothof said output circuits, means for controlling the attenuation in theoutput circuit of said radio receiving apparatus, means for controllingthe attenuation in the output circuit of said interphone communicationsystem, and means for mechanically coupling the above control meanstogether so that as the attenuation in one circuit is changed theattenuation in the other circuit is also simultaneously changed, wherebythe signal level in the sound reproducing device from the output circuitof said interphone communication system is affected primarily by theadjustment of its attenuation control means in its output circuit andsecondarily by the adjustment of the attenuation means in the outputcircuit of said radio receiving apparatus which changes the impedance ofthe shunt path formed by the attenuation means and said radio receiveroutput circuit.

12. In a communicating system having two sources of audio frequencysupply, a plurality of sound reproducing devices connected in parallelto the output circuit of both of said audio frequency supply sources, avariable audio attenuator connected in the output circuit of one sourceof supply to at least one reproducing device, said attenuator comprisingan impedance network of variable and fixed impedance elements which incombination with the reproducing device oiler a substantially constantimpedance load to said source of power supply so that the adjustment ofthe attenuation to one reproducing device will have no noticeable effectin the signal from the same source upon the other reproducing device,and means for maintaining the signal in said reproducing device fromsaid second source constant regardless of the adjustment of the variableattenuator.

13. The invention as defined in claim 12, wherein said last-named meanscomprises a variable attenuating resistor connected in the outputcircuit of said second audio frequency supply means.

14. In a communication system having two sources of audio frequencysupply, a plurality of sound reproducing devices, a plurality ofvariable audio attenuators comprising an-impedance network having fixedand variable impedance elements, means connecting a variable attenuatorin the output circuit of one of said supply means to each reproducingdevice, a plurality of isolating resistors connected in the outputcircuit of said other source of supply and in series with eachreproducing device, and means whereby the signal level in saidreproducing device from said other source may be adjusted so that itslevel will not be decreased with an increase in the signal level in thesame reproducing device from the first-named source.

15. The invention as defined .in claim 14, wherein said last-named meanscomprises an adjustable resistance element connected in series with eachreproducing device and isolating resistor.

16. In a communication system having two sources of audio frequencysupply, a sound reproducing device connected to both sources of supply,a variable attenuator connected in the output circuit of one source tosaid reproducing device, said variable attenuator comprising animpedance network having fixed and variable impedance elements, thevariable impedance elements each having adjustable contact arms, avariable impedance element connected in the output circuit of saidsecond source and in series with said reproducing device, saidvariableimpedance also having an adjustable contact arm, means wherebythe adjustable contact arms of each variable impedance element may bemoved together so that as the attenuation in the output circuit of onesource is increased,

the attenuation in the output circuit of the other source is alsoincreased.

17. In a communication system having two sources of audio frequencysupply, a plurality of sound reproducing devices, means connecting eachreproducing device across each source of supply, a variable attenuatorin the output circuit of one source of supply to at least onereproducing device, a variable resistor having an adjustable contact armin the output circuit of said other source of supply to at least thesame reproducing device, the variable attenuator comprising variableresistance elements having adjustable contact arms, and means wherebythe adjustable contact arms of all the variable resistor elements in theattenuation control for each reproducing device may be moved together sothat as the signal in the reproducing device from one source isincreased the signal in the same reproducing device from the othersource is decreased.

18. In a communication system for aircraft having a plurality of sourcesof audio frequency supply, a sound reproducing device, means connectingsaid sound reproducing device to each of said sources so that it maysimultaneously reproduce the signal from each, a pluralityof variableaudio attenuators, each comprising an impedance network having fixed andvariable impedance elements, one of said variable attenuators beingconnected in the output circuit of each supply source to the commonreproducing device whereby the output level from each source may beadjusted in the single reproducing device, a second sound reproducingdevice and means connecting said second sound reproducing device to theoutput circuit of one of said audio frequency sources, said meansincluding a variable audio attenuator and circuit arrangements whereby asignal from said other source in order to reach said second reproducingdeviceis subjected to the attenuation introduced in each output circuitby each variable attenuator.

19. In a communication system for aircraft having two sources of audiofrequency power supply, a plurality of sound reproducing devices, aplurality of variable audio attenuators, means including circuitconnections whereby one sound reproducing device may simultaneouslyreceive the signal from each audio frequency supply source through asingle variable audio attenuator, means including circuit connectionswhereby a second sound reproducing device may receive a signal from oneof said power sources through a single variable attenuator but thesignal from the other source is subjected to the attenuation introducedin each output circuit by each variable attenuator.

20. In a communication system for aircraft the combination of an outputcircuit of a radio receiving apparatus, an output circuit of aninterphone communication system, a sound reproducing device, conductorsconnecting said sound reproducing device and each output circuits inmultiple whereby the sound reproducing device and the output circuit ofsaid radio receiving apparatus comprise a multiple load to the outputfrom said interphone communication system, and a variable attenuatorwith a movable control means in the conductors connecting saidreproducing device to the output circuit of said radio receivingapparatus whereby an increase or decrease in the attenuator to the audiooutput of said radio receiving apparatus produces a correspondingdecrease or increase in the radio signal level in said sound reproducingdevice and at the same time produces an inverse effect in the interphonesignal level in said sound reproducing device by causing an increase ordecrease respectively in the impedance of the shunt circuit containingthe variable attenuator and the output circui of said radio receivingapparatus. 1

21. The invention as defined by claim 20 characterized further by theaddition thereto of a second variable attenuator with a movable controlmeans in said conductors connecting the sound reproducing device to theoutput circuit of the interphone communication system, and means formechanically coupling said movable control means whereby adjustmentproviding for an increase or decrease in the signal level from saidradio receiver output will cause an accentuated inverse change in thesignal level from said interphone communication system output by causinga decrease or increase, respectively, in the attenuation to'said soundreproducer.

. 22. The invention as defined by claim 20 characterized further by theaddition thereto of a second variable attenuator with a movable controlmeans in said conductors connecting the sound reproducing device to theoutput circuit of the interphone communication system, and means formechanically coupling said control means so that adjustment providingfor an increase or decrease in the signal level from said radio receiveroutput will compensate for the inherent inverse change in signal levelfrom said interphone communication system output whereby the said signallevel will remain substantially constant as the signal level from saidradio receiver output is increased or decreased.

' JAMES C. COE.

